Membrane bioreactors (MBRs) are widely used for wastewater treatment because of their improved performance resulting in better water quality and minimal space requirements compared to conventional treatment processes. MBRs include immersed porous membranes to extract clean water (i.e., permeate water) from waste that is mixed with a biomass including active aerobic organisms. This mixture of waste and biomass is generally called a mixed liquor. The immersed membranes generally include tubular hollow fiber membranes or plate-type filtration membranes. Permeate water is removed from the mixed liquor by passing through the pores of the membranes.
Membrane fouling remains a significant challenge for conventional MBRs. This is due to the accumulation of soluble and particulate materials from the mixed liquor onto and into the porous membranes. Fouling of the porous membranes leads to increased energy costs, poor operating conditions, poor efficiency, and frequent membrane replacement. Conventional treatments for cleaning the porous membranes include aeration, intermittent permeation, permeate backwashing, air backwashing, and chemical cleaning. Aeration generally involves streams of large, coarse air bubbles being provided at the base of the membranes in order to scour the fibers as they float upward toward the top of the membranes. Aeration (both coarse and fine bubble aeration) is also used to circulate the mixed liquor to help re-suspend solids. Due to the air bubbles only being released at the base of the membranes, dead zones deprived of sufficient cleaning or circulation are often present in conventional MBRs. The other conventional treatments for cleaning the porous membranes require periodically stopping filtration, which increases energy costs and decreases permeate flow.
Some conventional MBRs use moveable fibers to help mitigate the fouling problem. The moveable fibers are fixed only at one end so that they can sway and rub against each other to help reduce fouling. However, breakage is a problem with these moveable fibers due to higher mechanical stress at their fixed ends, as well as abrasion due to rubbing against each other. As a result, these moveable fibers must be reinforced, which increases membrane costs.